1. Field of the Invention
The present invention relates to a color conversion apparatus that converts input color signals to color signals suitable for an output device and relates to a color conversion method and a computer program product.
2. Description of the Related Art
Conventionally, development of the color matching system (CMS) for image data, which is performed when color image data on a computer is output to a color output device such as a color printer, has been actively pursued. The basic function of the CMS is to quantitatively measure (colorimetry) colors of image data represented by RGB signals and convert them to color signals, corresponding to the image data, used for an output device. However, because the color reproduction range performed by electrophotography or an inkjet printer is extremely narrow compared with that produced on a display, it is impossible to faithfully reproduce colors on the display. Accordingly, a technology for mapping colors that cannot be reproduced by the output device into reproducible colors (also referred to as a gamut mapping technology) is known, and various gamut mapping methods have been proposed.
For example, with the color gamut mapping method and the color gamut mapping apparatus disclosed in Japanese Patent Application Laid-open No. 2004-032140, a color gamut is mapped in such a manner that saturated colors of the primary and secondary colors of an input device are matched with saturated colors of the primary and secondary colors of an output device. Considering this, a technology for mapping an edge line of color gamut of the primary and secondary colors of the input device in such a manner that the edge line is converted into a continuous and smooth line is disclosed.
Japanese Patent Application Laid-open No. 2005-348053 discloses a technology for mapping, using a color conversion apparatus, points inside an output gamut after matching lightness of the maximum saturation point with the output gamut while maintaining the saturation.
Japanese Patent Application Laid-open No. 2003-143425 discloses a technology for mapping, using an image processing apparatus, performed by obtaining a target mapping of the maximum saturation point of an input gamut and correcting the input gamut in such a manner that the lightness of the target mapping matches the lightness of the maximum saturation point.
With the above-described technologies, input data is converted to color signals independent of devices, for example, L*a*b* signals that are CIELab space signals, and the target mapping is determined on a CIELab space. Uniform color space can be represented by three components, i.e., lightness, saturation, and hue, and moreover, a color difference that is defined as a distance in a space conforms well to a color difference perceived by a human. Accordingly, it is possible to perform mapping that is natural for human perception.
For example, for colors that cannot be reproduced by the output device, a technology for reproducing colors having a minimum color difference obtained by varying, from among colors reproduced by the output device, the weight of differences in lightness, saturation, and hue is known (see Japanese Patent Application Laid-open No. H-10-84487). Furthermore, a technology for performing compression mapping is also known, where target points for projection are set on an achromatic axis or on a saturation axis of a hue identical to a hue of input color signals, and compression mapping is performed within gamut of an output device by making the hue constant for colors that are out of gamut of an output device (see Japanese Patent Application Laid-open No. H9-168097, or Japanese Patent Application Laid-open No. H9-18727). However, with such gamut mapping technologies, there is a problem in that underexposure or inversion phenomenon of saturation may occur.
FIG. 11 is a schematic diagram explaining a state in which points P1 and P2 of an input gamut on a boundary are mapped, by a conventional color conversion apparatus, onto a boundary of an output gamut in such a manner that color difference is minimized in a hue plane while keeping the hue. FIG. 12 is a schematic diagram explaining a state in which the points P1 and P2 of the input gamut on the boundary are mapped, by a conventional color conversion apparatus, onto a boundary of the output gamut and toward target points projected on an achromatic axis.
As shown in FIG. 11, points P1′ and P2′, which are target mapping, are overlapped and there is no difference in tone between points P1 and P2. Furthermore, as shown in FIG. 12, difference in tone remains for the target mapping points P1′ and P2′ because these two points are separate. However, in terms of a change in saturation (in the lateral direction), the relation between the points P1 and P2 on the boundary of the input gamut is saturation (P1)>saturation (P2), whereas the relation between the target mapping points P1′ and P2′ is saturation (P1′)<saturation (P2′), resulting in inversion of the magnitude of saturation. This indicates the above-described problem of inversion phenomenon of saturation.
To solve this inversion phenomenon of saturation, the technology disclosed in Japanese Patent Application Laid-open No. 2004-032140 has been developed. Specifically, mapping is performed by obtaining the direction, in which the maximum saturation point of the input gamut is mapped onto the maximum saturation point of the output gamut, setting an intersection of the mapping direction and an achromatic axis as a target point for projection, and then mapping other points.
FIG. 13 is a schematic diagram explaining a technology of gamut mapping disclosed in Japanese Patent Application Laid-open No. 2004-032140. FIG. 14 is a schematic diagram illustrating a state in which gamut mapping similar to that used in FIG. 13 is used with respect to an output gamut that has a scooped shape in a shadow area.
As shown in FIG. 13, there is a difference in tone between the target mapping points P1′ and P2′, and no inversion phenomenon of saturation occurs; therefore, satisfactory gamut mapping is performed. However, in practice, the shape of the output gamut is sometimes not like the one that is shown in FIG. 13. Accordingly, assuming an output gamut having a scooped shape in a shadow area like that shown in FIG. 14, when gamut mapping similar to that used in FIG. 13 is performed, as shown in FIG. 14, there is a difference in tone between the target mapping points P1′ and P2′, and no inversion phenomena of saturation occurs; however, there is a risk of occurrence of tone jump because the difference in tone is too large. Thus, when an image is converted using a gamut mapping technology disclosed in Japanese Patent Application Laid-open No. 2004-032140, tones are considerably different between images before and after the conversion, which poses a problem.